A well-known mass-analyzing method for identifying a substance having a large molecular weight and for analyzing its structure is an MS/MS analysis (or tandem analysis). FIG. 15 is a schematic configuration diagram of a general MS/MS mass spectrometer disclosed in Patent Documents 1 through 3 or other documents.
In this MS/MS mass spectrometer, three-stage quadrupole electrodes 12, 13, and 15 each composed of four rod electrodes are provided, inside the analysis chamber 10 which is vacuum-evacuated, between an ion source 11 for ionizing a sample to be analyzed and a detector 16 for detecting an ion and providing a detection signal in accordance with the amount of ions. A voltage ±(U1+V1·cos ωt) is applied to the first-stage quadrupole electrodes 12, in which a direct current U1 and a radio-frequency voltage V1·cos ωt are synthesized. Due to the action of the electric field generated by this application, only a target ion having a specific mass-to-charge ratio m/z is selected as a precursor ion from among a variety of ions generated in the ion source 11 and passes through the first-stage quadrupole electrodes 12.
The second-stage quadrupole electrodes 13 are placed in the well-sealed collision cell 14, and Ar gas for example as a CID gas is introduced into the collision cell 14. The precursor ion sent into the second-stage quadrupole electrodes 13 from the first-stage quadrupole electrodes 12 collides with Ar gas inside the collision cell 14 and is dissociated by the collision-induced dissociation to produce a product ion. Since this dissociation has a variety of modes, two or more kinds of product ions with different mass-to-charge ratios are generally produced from one kind of precursor ion, and these product ions exit from the collision cell 14 and are introduced into the third-stage quadrupole electrodes 15. Since not every precursor ion is dissociated, some non-dissociated precursor ions may be directly sent into the third-stage quadrupole electrodes 15.
To the third-stage quadrupole electrodes 15, a voltage ±(U3+V3·cos ωt) is applied in which a direct current U3 and a radio-frequency voltage V3·cos ωt are synthesized. Due to the action of the electric field generated by this application, only a product ion having a specific mass-to-charge ratio is selected, passes through the third-stage quadrupole electrodes 15, and reaches the detector 16. The direct current U3 and radio-frequency voltage V3·cos ωt which are applied to the third-stage quadrupole electrodes 15 are appropriately changed, so that the mass-to-charge ratio of an ion capable of passing the third-stage quadrupole electrodes 15 is scanned to obtain the mass spectrum of the product ions generated by the dissociation of the target ion.
In a conventional and general MS/MS mass spectrometer, the length of the collision cell 14 in the direction along the ion optical axis C which is the central axis of the ion stream is set to be approximately 150 through 200 mm. In addition, the supply of the CID gas is controlled so that the gas pressure in the collision cell 14 is a few mTorr. However, when an ion proceeds in a radio-frequency electric field in the atmosphere of comparatively high gas pressure, the kinetic energy of the ion attenuates due to a collision with gas, so that the ion's flight speed decreases. In the collision cell 14 in the aforementioned conventional MS/MS mass spectrometer, since the decelerating area of the ion's kinetic energy is long, the delay of the ion is significant; a decelerated ion could stop in an extreme case.
In the case where an MS/MS mass spectrometer is used as a detector of a chromatograph such as a liquid chromatograph for example, it is necessary to repeatedly perform an analysis at predetermined intervals of time. Hence, if the ion's delay is significant as previously described, an ion which should normally pass through the third-stage quadrupole electrodes 15 might not be able to pass through it, which causes a degradation in the detection sensitivity. In addition, an ion remaining in the collision cell 14 may appear at a timing at which no ion should appear in reality, which causes a ghost peak. Moreover, since it takes time for an ion to reach the detector 16, the time interval of the repeated analysis is required to be previously determined in view of such a situation, which might cause an omission of analysis information in a multi-component analysis.
In order to avoid such a variety of problems as previously described, conventionally and generally, a direct current electric field is formed which has a potential gradient in the ion's passage direction in the collision cell 14, so that an ion should be accelerated by the action of the direct current electric field. However, even though such an acceleration is performed, in the conventional configuration, the time period for an ion to pass through the collision cell 14 is not negligible. In view of this, it is necessary to set the relatively low speed of the mass scan in the third-stage quadrupole electrodes 15, which is the subsequent stage, which takes time to collect the data for one mass scan. In the case where a direct current electric field having a potential gradient in the ion's passage direction is formed as previously described, the configuration of the electrodes themselves and that of the voltage application circuit are complicated compared to the case where a constant direct current electric field without a potential gradient is formed, which causes an increase in cost. Simultaneously, the configuration in which three-stage quadrupole electrodes 12, 13, and 15 are linearly arranged as previously described has a problem in downsizing the apparatus.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. H07-201304    [Patent Document 2] Japanese Unexamined Patent Application Publication No. H08-124519    [Patent Document 3] U.S. Pat. No. 5,248,875